Future El Niño events could spark high wildfire risk

A firefighter monitors a back fire while battling the Medocino Complex fire on August 7, 2018 near Lodoga, California. The Mendocino Complex Fire, which is made up of the River Fire and Ranch Fire, has surpassed the Thomas Fire to become the largest wildfire in California state history with over 280,000 acres charred and at least 75 homes destroyed.(Credit: Justin Sullivan/Getty Images)

As temperatures rise, wildfires may get worse in areas that already experience them and become more prevalent in areas where they’re not yet a big risk, a new study warns.

At roughly 415,000 acres, Northern California’s Mendocino Complex Fire is now the state’s largest recorded wildfire, surpassing the record held by Santa Barbara and Ventura counties’ Thomas Fire, which occurred less than a year before.

The trend of growing intensity and extremity of recent wildfires triggered scientists wondering how a warming Earth might affect El Niño/Southern Oscillation (ENSO) and how, in turn, that pattern might affect the likelihood and intensity of future wildfires to examine the issue in their new work.

Their findings could have implications on land use and on wildfire fighting and prevention strategies at urban/wildland interfaces, the researchers say.

Ripple effect

“This paper is really saying that in fire-prone places like California and Australia, we can expect future El Niño and La Niña events to have a bigger impact on fire risk in a given year,” says coauthor Samantha Stevenson, a faculty member at the Bren School of Environmental Science & Management at the University of California, Santa Barbara.

“That’s because the sensitivity of land temperature and precipitation to changes in tropical Pacific Ocean temperature is increasing due to climate change.”

Like many climate-based events, which take place over long timescales, tracking the ripple effects of a climate pattern can be difficult, given the many individual processes that can affect the result.

ENSO—the collective name for the ocean-warming El Niño weather pattern and its complementary ocean-cooling La Niña pattern—has particularly complex and extreme teleconnections. It drives drought in some places while promoting fertile vegetation in others. Food sources may dwindle in some regions, while fresh water supplies may be built in others. Throughout, the process affects human populations and their supports—such as agricultural production, energy use, and fire prevention.

More ‘bang for your buck’

For the study, which appears in Geophysical Research Letters, researchers used large “ensemble” sets of climate simulations, projecting future climate under a business-as-usual climate scenario assuming no major efforts to combat climate change. They then examined changes in the sensitivities of regional climate to ENSO-related sea surface temperature anomalies between the historical period and the late 21st century.

In addition to looking at changes in ENSO-driven temperature and rainfall patterns, one of the climate models (the Community Earth System Model, or CESM) includes a wildfire scheme that simulates the risk of fire activity based on factors such as soil moisture, fuel load, and applied statistical relationships between fire season length and burned area to simulate fire activity.

All of the quantities the researchers examined showed an increase in the future effects of El Niño and La Niña events—in other words, we can expect a larger ENSO-driven “bang for your buck” in the future.

“Typically what happens during an El Niño or La Niña event is you get changing atmospheric circulation patterns,” Stevenson explains.

These changes in atmospheric circulation patterns, which originate in the ocean as rising or falling sea surface temperatures, in turn can cause shifts in things like wind patterns, cloud cover, atmospheric temperature, and precipitation, which then affect conditions on land. These impacts will be intensifying in the future as the Earth warms, according to the researchers.

How strong will future events be?

“Future ENSO is having a larger impact on the land surface since the Earth is warming—and it’s easier for land to warm than the ocean in general, because it has a lower heat capacity—so the land is going to be evaporating more moisture,” Stevenson says.

Add an El Niño event on top of that, she adds, and the level of evaporation will increase. With drier soils and fuels, the likelihood and intensity of wildfires are also bound to increase.

Climate scientists still aren’t sure how strong El Niño and La Niña events themselves will actually be in the future, Stevenson says, but what they do agree on is that events of a given magnitude will cause larger impacts to wildfire risks as the Earth warms, worsening fires in areas that already experience them, and creating conditions for them where they aren’t currently a big risk.

Finding these pieces of the puzzle will be critical for future studies and wildfire fighting strategies, she says.